The purpose of chromatography is to determine the respective concentrations of various chemical constituents in matter being examined, e.g., drinking water, suspected toxic waste or blood. In gas chromatography, a sample of the matter having a known volume is injected into a stream of carrier gas as it enters a separation column. The column elutes the constituents in what are referred to as "peaks" at respectively different times from its other end. In each peak, the concentration of the chemical constituent in the carrier gas increases from zero to a maximum and then goes back to zero. The output of the separation column is applied to a detector that produces an electrical output signal corresponding to this peak of changing concentration, and integrating means are provided for deriving a signal corresponding to the area under each peak. The latter signals indicate the relative amounts of the different chemical constituents contained in the injected sample and a comparison with the areas of peaks produced by an injected sample having a known amount of a chemical constitutent indicates the actual amounts of the various constituents contained in the sample being analyzed.
Various types of detectors may be used, but a detector that provides an output signal corresponding to the thermal conductivity of gas contained within it is advantageous because of its universal sensitivity. As it would be difficult to make an accurate measurement of thermal conductivity, the detectors of this type provide a differential output signal related to the relative thermal conductivity of the mixture of constituent and carrier gas in a peak and the carrier gas by itself. For many years, this was done by utilizing a balanced Wheatstone bridge circuit in which two or four arms were essentially identical filaments respectively contained in different cavities within a large metal block and the other two arms were matched resistors. Electrical current was passed through the resistors and the filaments so as to make the temperatures of the filaments greate than that of the metal block. If the gases in the respective cavities had the same thermal conductivity, the rate of heat flow from each filament to the wall of its cavity would be the same so that the resistances of the filaments would still be equal and the bridge would remain in balance; but if the thermal conductivity of the gas in one cavity (which could be carrier gas) were different from the thermal conductivity of gas in the other cavity (which could be constituent gas), the resistances of the filaments would be different and tend to cause an imbalance in the bridge so as to produce an output signal proportional to the imbalance and therefore proportional to the relative thermal conductivities of the gases.
In order to obtain accurate results with such a detector, it is essential that the filaments, cell geometry and wall temperature be identical. The metal block acts as a heat sink having considerable thermal inertia so that the temperature of the walls of the cavities through which the respective gases flow does not change. Before analysis can begin, however, it is essential to operate the detector with carrier gas flowing through both cavities until the bridge becomes stabilized as far as temperature is concerned. Unfortunately, this may require as long as a full day. Drift due to chemical attack on the filaments is always present.
These difficulties were overcome by the thermal conductivity detector described by David E. Clouser and John S. Craven in their U.S. Pat. No. 4,254,654 entitled "Modulated Fluid Detector" which issued on Mar. 10, 1981. It employs a single filament and cell in one arm of a Wheatstone bridge and causes the gas flow through the cell to alternate a number of times during each peak of constituent gas between carrier gas an elutant from the separation column. This produces an output signal that alternates between a value corresponding to the thermal conductivity of carrier gas and a value corresponding to the thermal conductivity of the mixture of carrier and constitutent gas. The metal block is not required because the temperature of the wall of the single cell varies so slowly with respect to the rate of alternation as to have substantially the same effect on successive signal values. Synchronous detection techniques are employed to derive an output signal that is the integration of the difference between the signal value attained during one half of the alternation when the constituent gas in the cell and the signal value attained during an adjacent half or the alternation when carrier gas in the cell.
The respective alternate output signal values must obviously be obtained when the cell is full of carrier gas or mixture of constituent and carrier gas. The flow rate required to attain this condition depends on the desired rate of alternation and the cell volume. With the alternation rates of five to ten per second presently desired and with the cells of the smallest practicable volume, the required flow rate is about 5 to 10 sccm/minute, which exceeds that generally used with either narrow or wide bore open tubular columns so that, if the detector is to be used with them, it is necessary to use make-up gas; but no make-up gas is required when the detector is used with packed columns.